Doorbells are a great idea. They let you know when someone who's not a burglar is trying to enter your house, apartment, or squat. They eliminate the need for lots of noisy yelling and startling door pounding.

The one thing doorbells don't do is to let you know when someone forgets to press your doorbell—or doesn't press it hard enough. Critical packages, proselytizers, and girl scout cookies could be waiting idly at your steps right now

To solve this doorbell loophole, I decided to make a laser triggered doorbell alarm that rings whenever someone steps up to your door.

Materials

Laser module

LDR light sensor

Arduino

Piezo buzzer or relay

10k resistor

Step 1: Circuit

Setting up a laser tripwire circuit is wonderfully easy. All we need to do is power the laser and point it at the light sensor. The Arduino records the values from the light sensor and powers the buzzer when the beam is broken.

Connect the circuit as shown below.

Be sure to point the laser beam in the direction of the light sensor. Note that the LED below represents the laser module.

Step 2: Arduino

Once you have wired up the circuit, plug in your Arduino and open the programming environment. Disconnect the buzzer from the breadboard and upload this code. Your laser should already be on as it is connected directly to 5 volts.

Aim it at your light sensor and plug in your buzzer. Interrupt the beam with your hand and the buzzer will sound until you remove your hand.

Step 3: Installation

This setup works best for a door with a screen door or a porch with an outer door. Position your laser diode at the bottom of the door jam so that the laser shines up the length of the door as shown.

Position the light sensor at the top of the doorjamb and align it with the laser beam.

You can install the buzzer anywhere you like, though it would make sense to have a wire leading inside so you can hear the alarm ring.

If you are having a problem with sunlight interfering and setting off the bell, try putting the light sensor in a tube to block out all extraneous light. You could also hook up the buzzer pins to a relay to control a real doorbell!

Needless to say, this works great as a security system for keeping nosy siblings out of your room. You could even practice to be a diamond thief if you made a few modules and aligned them in a grid.

What do you think is the best application for this laser tripwire? Let us know in the comments below. Feel free to post questions in the comments or in the forum. We love hearing from you! Your wish is our command.